The present invention relates to cathodes for use in gas discharge lamps. More particularly, the invention relates to cold cathodes used in low pressure gas discharge lamps.
Gas discharge lamps operate by passing a flow of electrons through a gaseous environment. The electron current flow is driven by a voltage potential across the electrodes. Cold cathode type lamps rely on surface electron emission to provide the electrons and to initiate the flow of electric current. A relatively large voltage potential is required to initiate electron flow. The voltage required depends upon the density of the gas within the lamp. As commonly known, high pressure discharge lamps are filled with gas at atmospheric pressure or higher. Low pressure discharge lamps are filled with gas typically at 0.500 atmospheres or less. Further, electron flow is related to the surface area of the cathode.
It is known in the art to provide a cathode which is substantially cylindrical, thereby increasing the surface area available for electron emission. Also, to reduce the cathode's physical size and improve lamp efficiency, the inside of the cylinder is coated with a compound which is capable of enhancing the availability of free electrons. Such an emission assisting coating greatly enhances current flow per square inch of available electrode.
When electric current is passing through the gas in the lamp, some of the electrons will collide with atoms of the gas, passing energy to the atoms at different levels depending on the type of collision. In elastic collisions, the electrons essentially bounce off the gas atoms and transfer some of their kinetic energy to the gas atoms. In excitation collisions, the electrons collide with a gas atom in such a way that one or more of the atom's electrons are moved to a higher energy level but without leaving the atom's influence. Almost instantaneously, the displaced electron will return to its ground state, releasing the excess energy it picked up as the result of the collision, some of which is visible light. At still higher energy levels, the high velocity electrons collide with gas atoms with sufficient force to liberate one or more of the gas atom's electrons. The freed electrons become part of the electric current, and flow towards the positively charged anode. Positively charged gas ions are attracted to the negatively charged cathode, and combine with free electrons if they can.
Thus, some positively charged gas ions will exist in the envelope of the lamp. If they are sufficiently close to the negatively charged cathode, they will not have time to pick up an electron, and they will accelerate toward the cathode. The area of space where positive ions accelerate towards the negatively charged cathode is commonly known as the negative column. If the cathode of the lamp is a shell type with a sidewall forming an interior, then the negative column may be within the interior of the cathode shell. When the gas ions collide with the cathode, they have sufficient kinetic energy to vaporize a small part of the cathode material, causing gradual erosion of the cathode. This process of erosion is commonly known as sputtering.
Cold cathode type low pressure gas discharge lamps, commonly called neon lamps, are well known, popular, and attractive lamps for custom display applications. Unfortunately, lamps using such cathodes suffer from several disadvantages. First, they are expensive to produce because of time and energy consuming processing techniques. The lamps are sensitive to various sorts of contaminants, and such sensitivity necessitates the use of expensive processing techniques to minimize the amount of contaminants in the lamp after manufacturing.
The first of such techniques, commonly called bombarding, involves passing a current substantially larger than the operating current of the lamp through a partially evacuated lamp. The purpose of this procedure is to remove impurities such as moisture from the glass wall and to oxidize and thereby activate the emission-assisting coating of the cathode. Impurities such as moisture or oxygen can be harmful to the lamp. In addition, the oxidization of the emission coating of the cathode releases some carbon dioxide which can also be a harmful contaminant. After heating, the lamp is usually back-flushed with an inert gas to dilute or remove whatever harmful gases are left in the lamp envelope after the bombarding process. Subsequently, the operating gases are placed inside the lamp. Processing equipment for bombardment can be relatively simple, but in recent times, because of quality expectations, equipment has become more and more sophisticated, and thus more expensive. Further, the procedure requires a significant amount of energy and increases processing time.
An alternative process, known as oven pumping, is used less frequently. Oven pumping involves heating the glass in a specially designed oven under relatively high vacuum. Gases which evolve during the heating are continuously removed. Once the glass becomes relatively cold, the electrodes are heated with radio frequency heaters, flushed and back-filled. Again, this process adds time and expense to the manufacturing of gas discharge lamps.
Neither process guarantees a uniformly repeatable result. The final vacuum, and therefore the purity of the lamp environment, is dependent upon time, volume, and gas flow restriction. By their nature, cold cathode low pressure lamps are often intended for custom applications and therefore the results of such processes may vary considerably.
Such lamps also suffer from a reduced life span for various reasons. First, the active gas within the lamp may be lost. The vast majority of gas losses are due to absorption by the cathodes. When gas ions collide with the cathode at high velocity, they have a high probability of being accelerated into the cathode and embedding into the surface of the cathode. Second, the cathode itself may be eroded by sputtering. Such high velocity gas ions often transfer their kinetic energy to the metal causing a very high localized temperature, thereby vaporizing some of the metal particles of the cathodes which condense nearby. Such sputtering removes the emission assisting coating within the cathode as well as the metal of the cathode itself. The sputtering continues through the lamp's life until the cathode erodes to such a degree that it is incapable of providing the free electrons needed to maintain the current flow. Third, phosphors, which are required in order to radiate light in the visible spectrum, may combine with contaminants in the lamp, thereby reducing the radiation efficiency of the lamp.
Existing lamps address these problems by several means which are expensive or limited in efficacy. A gettering material, a material which may absorb gaseous contaminants, may be placed within the glass envelope of the lamp to remove some impurities which were not removed in the initial manufacturing. Such a getter is commonly a solid block, and therefore will be limited in efficacy by the surface area of the solid. More commonly, manufacturers simply utilize more elaborate and precise processes of removing contaminants. Such processes add expense and time to the manufacturing process. Additionally, they are limited in efficacy because contaminants are still found in the lamp, and because erosion of lamp components still takes place.